Many network architectures depend on a common, network-wide time reference. Network-wide time references may be provided by an external source where available, for example, on-board Global Positioning System (GPS) receivers may provide a network-wide time reference. However, some applications and environments do not allow network timing through the use of GPS. For example, sensor networks are often deployed in tunnels, caves, or underneath dense foliage. Accordingly, there is a need for synchronization of time in the absence of external time references.